In the words of Hank Hill, “You’re not making Christianity better, you’re just making rock and roll worse.” :)
I found this article to have a pretty interesting background and history on the origins of Christian rock as a musical genre. Unsurprisingly, as with so much of modern Evangelical Christian subculture, Christian rock came about as an alternative to secular music trying to reach "nonbelievers" with something more enticing and familiar than gospel music.
"Many historians trace the birth of Christian rock to the release, in 1969, of 'Upon This Rock.' It was an inventive concept album, by turns fierce and sweet, that was the work of a stubborn visionary named Larry Norman—the founding father of Christian rock." The article continues to explain that Norman and other evangelicals leveraged the Hippie Movement at the time to promote the transformative power of Christianity, even so far as to describe encounters with Jesus as the "ultimate trip," fueling the Jesus Movement. Over time, this became a key strategy of Christian churches in America—to insert Jesus-loving lyrics into songs with modern sounds to attract youth.
To get into your question on the sound of it, it's helpful to understand the perspective of many of these Christian rock artists. Eddie DeGarmo of DeGarmo & Key unapologetically believed this and shared in his memoir, “Christian music is a lyric-based genre. If you’re not passionate about delivering a message, this isn’t the scene for you...We always tried hard to come up with a title and a song that could end up being the theme to a summer youth camp and plastered on t-shirts everywhere." And the success of bands like DC Talk proved this as they infused their Christian lyrics into songs made to match the sounds and phenomenon of Nirvana in the early 90s.
That brings me to possibly the most poignant quote to answer your question:
The focus on lyrics exacted a cost, because it encouraged listeners and musicians alike to view music as a meaningless delivery system for meaningful words.
And as Christian rock grew, especially with the rise of record labels like Tooth & Nail that brought this genre into the Christian mainstream and some even into the secular mainstream, the money to be made became extremely apparent. As parents and youth pastors alike searched for any opportunity to bring youth into or keep their faith, the solution was Christian rock. Its focus on message over sound meant it could follow a basic formula that made it easier to produce in large quantities over quality. And because these Christian bands had Christian record labels, more and more got through the filter instead of having to compete with the cut-throat demands of secular labels. So there was no incentive to make something 'different' but rather to make more. This quantity over quality strategy also gave rise to Christian worship which follows a similar formula that can be easily repeated over and over by even the most amateur of musicians but gives people a spiritual experience by adding some nice sounds to a religious message.